Before I tossed my old header tank, I decided to saw it open to see what sort of structure was on the inside. It's a bit more complicated than I expected. Photos below. The right side of the tank is a honeycomb of plastic pockets. Water entry is limited by a small hole in the bottom of each cell, and a small hole in the top which prevents the cell from ever being completely full. This traps a bit of air, which is what allows fluid expansion without fluid loss. You'll notice that the air pockets are filled with black goop, which seems to be a combination of oil and algae. As you can see in the last photo, these cells never really fill. The new tanks have something rattling around inside. I expect its a silicate SCA (supplemental coolant additive), which gradually mixes with the coolant. A very interesting design.

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